Transcript of Response surface methodology for optimizing adsorption ...
DOI: 10.22104/AET.2017.505
Azam Nojavan, Parvin Gharbani*
Department of Chemistry, Ahar Branch, Islamic Azad University,
Ahar, Iran
A R T I C L E I N F O A B S T R A C T
Article history: Received 31 January 2017 Received in revised form
4 May 2017 Accepted 13 June 2017
In this research, Kaolin modified by Cetyltrimethylammonium bromide
is used as an adsorbent for the removal of Reactive Blue 21 from
aqueous solutions. Response surface methodology was used to study
the effect of independent variables, such as Reactive Blue 21 dye
concentration (20, 40, 60, 80 and 100 mg/L), time (10, 20, 30, 40
and 50 min), initial pH (2, 4, 6, 8 and 10) and modified Kaolin
dosage (0.05, 0.1, 0.15, 0.2 and 0.25 g/50 mL) on dye removal
efficiency from aqueous solutions. At the optimum conditions,
predicted removal of Reactive Blue 21 by modified Kaolin was
98.26%. The confirmatory experiment was conducted, which confirmed
the results by 94.42 % dye removal. Thus, the experimental
investigation and statistical approach enabled us to predict
Reactive Blue 21 removal by modified Kaolin. Also, the kinetics and
isotherm adsorption of Reactive Blue 21 onto modified Kaolin was
obeyed pseudo-second order kinetics and Langmuir isotherm.
Keywords: Isotherm Kaolin Reactive Blue 21 Response Surface
Methodology
1. Introduction
The wastewater of textile industries due to presence of a large
number of contaminants is causing major hazards to the environment
[1]. These colored compounds are inhibiting sunlight and reducing
the photosynthetic reaction [2]. Since many organic dyes are
harmful to human beings, the removal of color from waste effluents
is important [3]. Nowadays, consumption of organic dyes in textile
industry is being increased and discharge of dye effluents in
environment will lead to damage of environment [4]. Dyes are
organic compounds that are resistant to degradation and due to
contain aromatic rings in their chemical structure, their
biodegradability is low [5-7]. They are toxic and cancerous and may
disrupt of kidneys, brain and central neural system in human [8,9].
Dyes are used extensively in textile, paper, plastic, food and
cosmetic industries [10]. Presence of dye wastes in environment
causes extensive pollution and production of byproducts via
oxidation, hydrolysis and other chemical reactions [11]. Different
physical (membrane filtration, microfiltration, ultrafiltration,
adsorption, coagulation and sediment) and
chemical (biological methods and advanced oxidation processes)
methods have been used to remove of days [12]. Among these methods,
adsorption is mostly used due to cost-effectiveness and simplicity
[13-15]. Activated carbon is a high capacity adsorbent for the
removal of pollutants [16], but it is expensive. So, the attention
is focused to finding a non-expensive and efficiencies adsorbents
such as clays (bentonite, sepiolite montmorillonite, alunite, and
kaolinite) [17,18]. Clays have been widely studied because they are
cheaper than other sorbents such as activated carbons and resins
[19]. To improvement of clays efficiency, modification of them is
down by various organic/inorganic compounds [20]. Results showed
that modification of the clay increased the adsorption capacity
[21].Modification is carried out using adsorption of a cationic
surfactant onto the external surface and into the interlayer
spacing of the clays [22], so the surface of clay changes from
hydrophilic to hydrophobic and from negatively to positively
charge. The sorption of a cationic surfactant onto the surface of
the natural clay is mainly governed by cation exchange and
hydrophobic interactions [23]. Kaolin clays were modified with
tri-polyphosphate [24,25], and 2-
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mercaptobenzothiazole [26]. Cetyl trimethylammonium bromide (CTAB)
has been used as surfactant for modifying the surface of natural
clays and zeolites [27]. Surfactants are surface active compounds
and capable of reducing surface and interfacial tension between
liquids, solids and gases [27]. It was found that very few studies
have been devoted to modify the surface properties of kaolin with
surfactants. Phthalocyanine reactive dyes are mostly used in dye
& textile industries. They are metal complexes that are used in
creations of blue and green colors [28]. Such dyes are mostly
derivatives of copper Phthalocyanine (such as Reactive Blue 21) and
they are toxic and dangerous due to presence of copper in their
structure. Thus low concentration of these dyes is harmful for
health of organisms especially human [28]. Therefore, it is
necessary to remove such compounds from aqueous environments.
Reactive Blue 21 (RB21) is a dye that widely used for coloring
cotton, wool, silk and polyamide textiles and contains copper
phthalocyanine chromophore vinyl- sulfonic acid, which is toxic
even in low concentrations [29- 31]. The mean IC50 value for the
blue dye was 278mg/L and the mean IC20 value was112 mg/L [32].RB21
has been removed from wastewater by various methods such as turnip
peroxidase [28], adsorption onto clinoptilolite [33] and Sorption
and Solubilization in Micellar Media [34]. In present research,
reactive blue 21 (RB21) has been chosen as a pollutant dye (Figure
1) and its removal by modified Kaolin with Cetyl trimethylammonium
bromide (CTAB) will be studied by Response surface
methodology.
Fig. 1: Chemical structure of RB21 dye
Adsorption isotherms are mathematical equations that are used to
determine amount of adsorbed substance on adsorbent in low
temperature. Langmuir(1), Freundlich(2) and Temkin(3) isotherms are
the most common isotherms and their equations have been stated as
follows,
respectively:
= ln () (3)
where, qe and qm are the amount of absorption per unit mass of
adsorbent at equilibrium (mg/g) and for complete coverage or a
single layer of absorbent absorption capacity (mg/g), respectively.
Ce is the solution equilibrium concentration (mg/l)B and KL is the
adsorption equilibrium constant (L/mg) of Langmuir. KF and 1/n are
Frundlich isotherm constants. B is a constant that depends on the
heat of adsorption and KT is the equilibrium constant of Temkin
isotherm (L/g) [12].
One of important studies in adsorption process is to study the
adsorption kinetics. Adsorption kinetics depends on physico-
chemical properties of adsorbent and it influences on adsorption
mechanism. In current research, pseudo first and second-order
kinetics have been studied to investigate adsorption kinetics of
RB21 onto K-CTAB.
The pseudo-first-order differential form is as follows [13]:
= (1 − −1) (4)
In the above equation, K1 is the first-order apparent speed
constant (min-1), qe and qt are the adsorption capacity in the
equilibrium and any time, respectively (mg/g). The
pseudo-second-order differential form is as follows:
=
1 + 2 (5)
In the above equation, K2 is the second-order apparent speed
constant (g mg-1 min-1) [12].
Design of experiments is a knowledge by which effectiveness of each
factor affecting process and output specifications can be stated in
form of an equation. Main goals of experimental design are
reduction of number of experiments, reduction of costs and
determination of variables that are the most effective on the
response. Other goals of experimental design are deletion of
unnecessary factors, calculation of importance percent of each
variable and determination of favorable condition [14]. Response
Surface Methodology (RSM) is a set of statistical techniques and it
is used for optimization of processes and the response is affected
by number of variables. Number of experiments is reduced and all
coefficients of second order regression model and effect of factors
are calculated by such technique. RSM is a helpful method to find
favorable state of factors and to shows the effect of factors on
results of experiments [15,16] Central composite designs (CCD) are
the mostly used designs in response procedure method. Such designs
are produced by combination of a complete two-level factorial
design, star design and a central point. Sometimes a number of
experiments are repeated in them. Therefore, N=2f+2f+1 experiments
are required for testing f factor. When the number of factors is
above three factors, this design is cheaper than three-level
factorial designs and
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requires less time. Points of complete factorial design are located
in -1 and +1 surfaces, points of star designs are located on (- α)
and (+α) surfaces and central points are located on the surface
[17,18].The mathematical relationship between response and
variables can be presented in an equation (6) and in the form of a
second- order polynomial:
2
0
i i i j
(6)
Here, y represents the predicted response for the elimination
efficiency, β0 is constant factor, βi is coefficient of linear
effects, βii is coefficient square effects, βij is factor
interactions, while xi and xj are variables. ε is the random error
between predicted and measured values [14].
2. Materials and methods
2.1. Materials
Reactive Blue 21 dye (M=159.62 g/mol, λmax= 614 nm) was purchased
from Ardabil Arta Tejarat Zarrin Company (Iran) and Kaolin was
purchased from Dae Jung Company (Korea). Cetyl trimethylammonium
bromide (CTAB), Sodium hydroxide and hydrochloric acid were
purchased from Merck Company (Germany).
2.2. Analysis
FTIR model BRUKER-TENSOR 27 (Germany), XRD model D5000 (Siemens Co.
Germany) and TGA model 1500, Rheometric Scientific were used to
identify modified Kaolin. Also, UV-Vis spectrophotometer model
DR5000-15V (HACH CO, America) was used to record dye concentration
at any time.
2.3. Modification of kaolin by CTAB
In order to prepare modified Kaolin, 10 g Kaolin and 90 ml
distilled water was added in 250 ml Erlenmeyer on the stirrer.
Then, 1 g CTAB was added gradually and it was stirred for 3h in
ambient temperature. Then, precipitate was filtered, washed several
times with distilled water and placed in an oven for 3h in 105
oC.
2.4. Procedure
RSM was used to obtain main effects of independent variables
affecting on the response in adsorption process of RB21 dye on
K-CTAB. The study is a central composite design with second order
model. In this research, effect of four independent factors on
response was studied including initial concentration of RB21 dye,
adsorbent dosage, time and pH. The levels of such factors have been
shown in Table 1.
Table 1. Factors and levels of the operational parameters.
Variables -2 -1 0 1 2
dosage (g/50 ml) A 0.05 0.1 0.15 0.2 0.25
Dye conc. (mg/L) B 20 40 60 80 100
pH C 2 4 6 8 10
Time (min) D 10 20 30 40 50
After entering factors and levels in Design-Expert 7 (DX7)
software, 30 experiments were suggested in different conditions. At
first, solutions were prepared with required concentrations and
their pH was adjusted in required ranges. The solution was poured
in 100 ml Erlenmeyer and was placed on the stirrer. desired amounts
of adsorbent were added to solution and it was Stirred for certain
times. Then, samples were filtered and their adsorption was
recorded by UV-Vis spectrophotometer. Experiments and their
predicted results is shown in Table 2.
Table 2. Designed experiments along with experimental and predicted
values.
Trial No
A (g/50ml)
B (mg/L
3 28.42
1 90.96
4 68.36
6 93.66
3 49.19
5 96.58
6 80
15 0.1 80 8 40 49.5 6
46.84 16 0.15 60 6 10 99.1 90.14 17 0.2 80 8 40 49.1
5 52.84
1 64.94
4 84.95
1 73.03
8 99.3
1 99.3
28 0.15 60 10 30 99.4 99.3 29 0.15 60 6 30 98.9
4 99.3
99.3
In this research, Removal%, qe and qt were obtained via following
equations:
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%Removal = ( 0 − )
(9)
where, Removal% is percent of dye removal, C0 ( mg/L) is initial
solution concentration, Ce ( mg/L) equilibrium concentration, Ct
(mg/L) concentration in any time, V is solution volume ( L) and M
is adsorbent mass (g) [12]. qe and qt are the adsorption capacity
in the equilibrium and any time, respectively (mg g-1).
3. Results and discussion
3.1. Characterization of modified Kaolin
Modified Kaolin was identified using FTIR, XRD and TGA. FTIR
Spectra of Kaoline and K-CTAB have been shown in Figure 2. Figure.
2a shows FT-IR of Kaolin. The peak
observed in 3674 cm-1 associates with OH attraction of internal
crystalline hydroxyls, in 3374 cm-1 related to H-O- H vibration of
adsorbed water and the peak of 2925 cm-1 associates with attraction
of C-H group. Peak of 1670 cm-1 associate with H-O-H bonds of
water. 1016 cm-1 and 632 cm-
1 peaks associate with functional groups related to Si-O and Al-OH.
Peak of 844-949 cm-1 shows bond vibrations of Al- OH group in
Kaolin and peak of 791 cm-1 associates with internal
poly-tetrahedral bonds of Si-O-Si in SiO2 [35]. Fig. 2b shows
Kaolin modified with CTAB. According to Fig. 2b, a pair of peaks
has been created in areas of 2850 cm-1 and 2921 cm-1 and they
associate with symmetrical and unsymmetrical stretching vibrations
of methyl and methylene groups [36]. Figure 3a shows XRD pattern of
Kaolin. It shows diffraction content of crystalline layered network
of Kaolin with impurities of quartz (Q) and muscovite (M). Peaks
observed in 2θ= 10, 26, 36, 38, 50° associate with Kaolin, 2θ=
19.31° associates with muscovite and 2θ= 20.91, 29.12° associates
with quartz. Fig. 3b shows XRD pattern of Kaolin modified with
CTAB. As shown, the intensity of Kaolin peaks has changed after
modification.
Fig. 2. FTIR of: a) Kaolin; b)K-CTAB
Fig. 3. XRD of a) K, b: K-CTAB
The thermal Gravity of Kaolin (K) and Kaolin-CTAB (K-CTAB) is
presented in Figure 4 As shown, three stages is observed in thermal
decomposition of K-CTAB . The first stage that
was occurred over 130C - 200C related to adsorbed water. The second
stage was down in the range of 300°C - 500°C due to the coordinated
water and the partial loss of organic
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moieties. The third loss was occurred due to the elimination of the
organic groups and the dehydroxylation of the silanol groups on the
clay surface over a temperature range of 550°C - 900°C. The typical
hydrophilic character of K, which was evident by the strong
difference between the percentages of the first mass loss stage. An
additional change was the increase inthe total mass loss, which
values were 11.49% and 13.79% for K and K-CTAB respectively.
Fig. 4. TGA analysis of kaolin (K) and Kaolin-CTAB (K-CTAB)
3.2. Contact time
In order to study equilibrium time, 50 ml of dye solution with
certain concentration was poured in Erlenmeyer. Then, 0.15 g of
K-CTAB was added in Erlenmeyer and the stirrer. Sampling was down
each 10 minute until 120 minutes. After filtration of samples,
adsorption of all samples was recorded using UV-Vis
spectrophotometer. Figure 5 shows removal percent of RB12 dye on
K-CTAB in different times. According to Figure 5, removal percent
was reached to equilibrium after 30 minutes.
3.3. Adsorption isotherms
In order to study adsorption isotherms of RB21 dye on k- CTAB,
solutions with different initial concentrations were prepared in
constant pH (6.2) and adsorbent and removal% were recorded at
equilibrium time (30 minutes). Results of non-linear diagrams of
adsorption of RB21 onto k-CTAB have been shown in Figure 6.
According to Figure 5, adsorption of RB21 onto K-CTAB is obeyed of
Langmuir isotherm. So, it can be concluded the adsorption of RB21
dye onto k-CTAB is occurred as a mono- layer [17].
3.4. Kinetics
Kinetics results of RB21 adsorption onto K-CTAB has been shown in
Figure 7.
As shown in Figure 7, experimental data are best fitted by pseudo
second order kinetics. Therefore, adsorption kinetics of RB21 onto
K-CTAB is pseudo second order.
Fig. 5. Effect of contact time on removal of of RB21 on
K-CTAB.
[RB21]0=60 mg/L; K-CTAB=0.15 g/50ml, pH=6.21.3.
Fig. 6. Langmuir, Frundlich and Temkin adsorption isotherms of RB21
dye onto K-CTAB, K-CTAB =0.15 g/50 mL, pH = 6.2, Time= 30min
Fig.7. Pseudo first and second order plot of RB21 adsorption onto
k-CTAB. Mg/ L 60 = [RB21], pH = 6.2 0.15 g/50 mL =K-CTAB
3.5. Response Analysis
To study the significance and adequacy of the model (Table 3),
Analysis of Variance (ANOVA) was applied. The results show a high
value of coefficient of determination (Pred. R2
0
20
40
60
80
100
R e m
q t
q t
Time (min.)
First order
second order
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= 0.9320 and Adj R2=0.9052).The Model F-value of 69.44 confirmed
the significant of model and there is only a 0.01% chance that
model occurs due to noise. In ANOVA Table,
values of "Prob>F" which are less than 0.0500 show the
significant of model terms. In this case B, C, D, AB, AC, BC, BD,
CD, A2, B2, C2, D2 are significant model terms.
Table 3. ANOVA for response surface quadratic model.
Source Sum of square Df Mean square F-value P-value
Model 109886.37 14 784.74 69.44 < 0.0001 Significant
A: K-CTAB (g/50ml) 7.32 1 7.32 0.65 0.4337
B: Dye Conc (mg/L) 268.6 1 268.6 23.77 0.0002
C: pH 193.4 1 193.4 17.11 0.0009
D:Time (min) 307.09 1 307.09 27.17 0.0001
AB 3517.38 1 3517.38 311.24 < 0.0001
AC 490.51 1 490.51 43.4 < 0.0001
AD 8.66 1 8.66 0.77 0.3952
BC 1387.38 1 1387.38 122.76 < 0.0001
BD 299.6 1 299.6 20.32 < 0.0001
CD 83.4 1 83.4 7.38 0.0004
AA 3527.65 1 3527.65 312.14 0.0159
BB 1312.31 1 1312.31 116.12 < 0.0001
CC 129.1 1 129.1 11.42 < 0.0001
DD 622.96 1 622.96 55.12 0.0041
Residual 169.52 15 11.3
Lack of fit 167.94 10 16.79 53.09 0.0002 Not significant
R2=0.9320 Adj-R2=0.9052
A proper mathematical model between independent variables (initial
concentration of RB21, adsorbent dosage, time and pH) and response
was obtained. The model by which data were statistically calculated
was second order mathematical model. The obtained equation between
response (RB21 dye removal %) and independent factors is shown as
following:
R(%)=106.92667+105.96667×Dose+1.98333×[RB21]
+17.01500×pH+4.81783×t+14.82687*Dose× [RB21]+55.36875×Dose
×pH+1.47125×Dose ×t- 0.23280× [RB21] ×pH-0.018941× [RB21]
×t-0.11416×
pH×t-4536.29167×Dose2-0.017292× [RB21]2-
0.54237×pH2-0.047657×t2
(7)
Coefficients of A, B, C, D parameters were obtained by regression
of linear effects, coefficients of mutual effects of parameters
(AB, AC, AD, BC, BD, CD) by regression of interaction between
parameters and coefficients of A2, B2, C 2 and D 2 by regress ion
of power ef fects of 2 . In order to investigate validity of
studies, residual values (the difference between experimental and
predicted responses) are obtained by frequency percent (normal
distribution (8a) and they have been shown in terms of the number
of experiments (8b). Linear curve of normal distribution for
residual values suggests accuracy of the model and randomization of
residual distribution shows precision of the model.
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Fig. 8. (a) Normal plot of residuals; (b) Residuals versus run
number
In order to investigate the integrated effect of the factors on
removal of RB21 by Kaolin, the RSM was used and 3D plots (Figure 9)
are shown in Figure 9. According to Figs. 9a, 9b and 9c, removal%
is increased by the increasing dosage of adsorbent due to increase
of surface area and then decreased. It may be due to agglomeration
of K-CTAB particles in larger amounts that resulting in reduction
of surface area and so decrease of Removal% [37]. As Figures. 9a,
9d and 9e, removal percent is increased by the increasing
concentration of RB21 dye because of forces applied on adsorption
sites via dye molecules but in higher concentrations of RB21 dye,
removal percent is reduced due to saturation of adsorptions sites
[38]. According to Figs. 9b, 9d and 9f, removal percent is
increased by increasing of pH. It is known that, pH of solutions is
an effective parameter on dye removal. In current research, pHzpc
of K-CTAB is obtained about 9.5 (not shown here). In fact, in
pH<pHzpc, adsorbent surface has positive charge and in
pH>pHzpc, adsorbent surface has negative charge. In strong
acidic media, production of protonated species lead to change of
RB21 dye structure thus removal percent is reduced. In strong
alkaline pHs, repulsion force between anionic dye and negative
charge adsorbent leads to repulsion and removal percent is reduced.
Therefore, the highest removal percent takes place in weak acidic
and alkaline media [39]. According to Figures. 8c, 8d and 8f,
increase of contact time has positive effect on dye removal percent
and it is due to increasing of contact time between adsorbent and
dye molecules that resulting in increasing removal percent of RB21
dye.
3.6. Validation
The results obtained from RSM based experimental trials were
validated by carrying out an independent run at a maximum pH of 4
with an initial RB21 concentration of 100 mg/L and 0.24 g/50 ml of
adsorbent at half an hour. A maximum removal of 94.42 percent was
attained which validated the design.
4. Conclusions
Natural Kaolin is not performance adsorbents to remove of dye
containments. To improve of its efficiency, Kaolin was modified by
large organic cations, such as cetyl trimethylammonium bromide in a
simple way. Results of XRD, TGA and FTIR confirmed modification of
Kaolin by CTAB. K- CTAB showed a higher adsorption capacity toward
RB21 than kaolin. Results showed that adsorption of RB21 dye onto
K-CTAB is obeyed Langmuir isotherm and pseudo second order
kinetics. As data, the removal of RB21 dye on K-CTAB is consistent
with the percent predicted by the Experimental Design. Therefore,
RSM is able to present a proper model to remove RB21 dye by
K-CTAB.
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Fig. 9. Counter plots of removal efficiency (%) as a function of
(a) initial RB21 concentration and adsorbent dosage; (b) adsorbent
dosage and pH; (c) adsorbent dosage and contact time; (d) initial
RB21 concentration and pH; (e) initial RB21 concentration and
contact time; (f) pH and contact time
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